On 10/19/18 10:06 AM, Michal Hocko wrote: > On Thu 18-10-18 19:11:47, Andrew Morton wrote: >> On Wed, 26 Sep 2018 16:22:27 +0200 Michal Hocko <mhocko@xxxxxxxxxx> wrote: >> >>>> MPOL_PREFERRED is handled by policy_node() before we call __alloc_pages_nodemask. >>>> __GFP_THISNODE is applied only when we are not using >>>> __GFP_DIRECT_RECLAIM which is handled in alloc_hugepage_direct_gfpmask >>>> now. >>>> Lastly MPOL_BIND wasn't handled explicitly but in the end the removed >>>> late check would remove __GFP_THISNODE for it as well. So in the end we >>>> are doing the same thing unless I miss something >>> >>> Forgot to add. One notable exception would be that the previous code >>> would allow to hit >>> WARN_ON_ONCE(policy->mode == MPOL_BIND && (gfp & __GFP_THISNODE)); >>> in policy_node if the requested node (e.g. cpu local one) was outside of >>> the mbind nodemask. This is not possible now. We haven't heard about any >>> such warning yet so it is unlikely that it happens though. >> >> Perhaps a changelog addition is needed to cover the above? > > : THP allocation mode is quite complex and it depends on the defrag > : mode. This complexity is hidden in alloc_hugepage_direct_gfpmask from a > : large part currently. The NUMA special casing (namely __GFP_THISNODE) is > : however independent and placed in alloc_pages_vma currently. This both > : adds an unnecessary branch to all vma based page allocation requests and > : it makes the code more complex unnecessarily as well. Not to mention > : that e.g. shmem THP used to do the node reclaiming unconditionally > : regardless of the defrag mode until recently. This was not only > : unexpected behavior but it was also hardly a good default behavior and I > : strongly suspect it was just a side effect of the code sharing more than > : a deliberate decision which suggests that such a layering is wrong. > : > : Moreover the oriinal code allowed to trigger > : WARN_ON_ONCE(policy->mode == MPOL_BIND && (gfp & __GFP_THISNODE)); > : in policy_node if the requested node (e.g. cpu local one) was outside of > : the mbind nodemask. This is not possible now. We haven't heard about any > : such warning yet so it is unlikely that it happens but still a signal of > : a wrong code layering. Ah, as I said in the other mail, I think it's inaccurate, the warning was not possible to hit. There's also a slight difference wrt MPOL_BIND. The previous code would avoid using __GFP_THISNODE if the local node was outside of policy_nodemask(). After your patch __GFP_THISNODE is avoided for all MPOL_BIND policies. So there's a difference that if local node is actually allowed by the bind policy's nodemask, previously __GFP_THISNODE would be added, but now it won't be. I don't think it matters that much though, but maybe the changelog could say that (instead of the inaccurate note about warning). Note the other policy where nodemask is relevant is MPOL_INTERLEAVE, and that's unchanged by this patch. When that's addressed, you can add Acked-by: Vlastimil Babka <vbabka@xxxxxxx> (Note I also agree with patch 1/2 but didn't think it was useful to formally ack it on top of Mel's ack supported by actual measurements, as we're all from the same company). > : Get rid of the thp special casing from alloc_pages_vma and move the logic > : to alloc_hugepage_direct_gfpmask. __GFP_THISNODE is applied to > : the resulting gfp mask only when the direct reclaim is not requested and > : when there is no explicit numa binding to preserve the current logic. > : > : This allows for removing alloc_hugepage_vma as well. > > Better? > >> I assume that David's mbind() concern has gone away. > > Either I've misunderstood it or it was not really a real issue. >
